This invention relates in general to the manufacture of cement clinker, particularly in rotary kilns. More specifically, the invention relates to the introduction of waste materials into the clinker production process.
The details of a typical cement pyroprocessing operation are well known. One type of rotary kiln for manufacturing Portland cement is depicted in FIG. 1. Air and a primary fuel, such as coal, are injected into the rotary kiln 10 and are combusted to supply heat energy. Wet or dry raw materials, known as raw mix, for producing cement, such as limestone, clay and sand, are injected into the feed end 12 of the kiln. The kiln is inclined so that as the kiln rotates, the raw materials move through the kiln counter-current to the direction of the flow of the hot combustion gases so that the raw materials are subjected to progressively higher temperatures. For instance, at the input end, a pre-calcining zone can be provided that has a gas temperature of about 1000° F. (538° C.). The kiln gas temperature can be increased to about 1600° (871° C.) in a calcining zone where the CaCO3 in the raw materials is decomposed. The calcined material then passes to a clinkering zone where it faces the burning zone temperature inside the kiln, approximately 2732° F. (1500° C.). It is in this zone that the feedstock is converted into the typical cement compounds, such as tricalcium silicate, dicalcium silicate, tricalcium aluminate, etc. A cooling zone follows at the output end 14 of the kiln. The resulting compound, or clinker, is later mixed with other materials, such as gypsum, and then finely ground to produce Portland cement.
In some clinker production facilities, a pre-calciner 20 is added at the feed end 12 of the kiln 10. A primary burner 26 and side burners 28 heat the gas within the pre-calciner and raise the temperature inside the vessel to approximately 860-900° C. The pre-calciner operates to dry, pre-heat and decarbonate (i.e. reduce the CaCO3 to CaO and CO2) the raw feed being provided to the kiln, which reduces the thermal load on the kiln and allows for the use of a shorter kiln tube. The pre-calciner 20 includes an inlet 22 for receiving raw materials and an outlet 24 for discharging combustion gas to appropriate conditioning equipment. In the typical facility, the raw materials introduced at the inlet 22 includes raw mix obtained from a hopper 30. As explained above, the raw mix includes calcaneous or clinker-forming materials, such as limestone.
It can be advantageous to burn waste materials in cement kilns, for several reasons. Such wastes would otherwise have to be disposed in a landfill or other long term containment, or incinerated as a means of destroying the materials. Landfill disposal typically is more expensive and less desirable than disposal by recovering the useful energy value of the waste. While these wastes provide energy to the kiln system, the kiln operator typically charges a “tipping fee”, or service charge for accepting and disposing of the waste. The tipping fee is charged because there usually is a cost for handling and/or for pollution control associated with the use of diverse waste streams. Thus, use of waste-derived fuel in a cement kiln provides a benefit to the fuel user and to the waste generator. Namely, the kiln operator may gain significant income from tipping fees as well as fuel value that reduces the demand for conventional fossil fuels, and the waste generator may have access to a lower cost disposal option for the waste. The environment also benefits from use of waste as fuel, because cement kilns have efficient destructive capacity for various wastes as fuel and resultant fuel combustion products, due to high burning zone temperatures and long retention times of materials in the high temperature zone. Valuable landfill space is conserved, fossil fuels are conserved, and wastes that might have contaminated land or water are efficiently destroyed.
Types of waste that have been used as fuel or that have been recycled or processed in a variety of high temperature kiln situations, including cement kilns, according to the prior art include waste tires, either whole or when reduced in size by some means (U.S. Pat. No. 5,473,998); hazardous waste liquids, or solids or both (U.S. Pat. No. 5,454,333); agricultural waste, for example rice hulls; paper mill sludge (U.S. Pat. No. 5,392,721); soil, sludge, sand, rock or water contaminated with organic solvents and/or toxic metals (U.S. Pat. No. 4,921,538); sewage sludge (U.S. Pat. No. 5,217,624); petroleum refinery sludge (U.S. Pat. No. 5,141,526); various hazardous combustible wastes (see U.S. Pat. No. 5,454,333 or U.S. Pat. No. 4,984,983) and non-hazardous low-grade fuel wastes such as wood, paper and chemical waste (U.S. Pat. No. 5,336,317).
The usual locations for the input of fuel, air and raw mix are at the opposite ends of the kiln. In addition, flue gases escape at the elevated feed end of the inclined rotary kiln tube. Waste-derived fuel is sometimes added as a supplemental fuel at a mid-kiln location. The temperature at this mid-kiln location is high enough to assure substantially complete combustion of the waste so that the kiln can derive fuel value from the waste material. Waste type fuels that have been introduced at a mid-kiln location include hazardous waste materials, as disclosed in U.S. Pat. No. 6,050,203, and whole or shredded tires, as described in U.S. Pat. No. 6,213,764.
There is a consistent need to improve the efficiency and yield of the cement clinker production process. Moreover, there is a continuing over-riding need to efficiently dispose of waste of all types, including solid waste materials. These needs can merge into an over-arching goal of improving the manner in which waste and refuse materials are used in the production of cement clinker.